1. Field of the Invention
The present invention relates to a semiconductor device and its manufacturing method, and more particularly to a semiconductor device which uses a ferroelectric film and its manufacturing method.
2. Description of the Related Art
In a semiconductor device such as a ferroelectric random access memory (FeRAM) which uses a ferroelectric capacitor using a ferroelectric film, if hydrogen or moisture penetrates the capacitor during the manufacturing process after the ferroelectric capacitor is formed, the characteristics of the ferroelectric capacitor, especially the polarization characteristics of the ferroelectric film, are degraded. To prevent this degradation, the capacitor is covered with a hydrogen diffusion preventive film, e.g., an aluminum oxide (Al2O3) film, to prevent the penetration of hydrogen. The Al2O3 film is preferably formed in an atmosphere which does not contain hydrogen, thus sputtering is used for the formation typically. However, miniaturization of the semiconductor device has increased the ratio of the height of the ferroelectric capacitor to space (aspect ratio). Because of the need to cover the ferroelectric capacitor with the large aspect ratio, sputtering which is poor in step coverage cannot be used as it is. According to a method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-68987, to improve the step coverage, a ferroelectric capacitor is covered with an interlevel insulator to reduce the step of the capacitor, then Al2O3 film is formed thereon by sputtering. As another technology, Jpn. Pat. Appln. KOKAI Publication No. 2003-51582 discloses a method of using a hard mask which has the hydrogen barrier characteristics in an etching process of a ferroelectric capacitor. According to the method, Al2O3 film that is a hydrogen barrier film is deposited by sputtering after the ferroelectric capacitor is formed. Therefore, coverage of a ferroelectric capacitor side face with the hydrogen barrier film is not really sufficient. Additionally, a method of forming an Al2O3 film by atomic layer deposition (ALD) having excellent step coverage has been put to practical use. However, ALD is not suitable to form a thick Al2O3 film because its deposition speed is slow.
In all of the aforementioned technologies, an interlevel insulator is formed on the hydrogen barrier film which covers the ferroelectric capacitor, i.e., Al2O3 film, and a contact plug is formed in these films being connected to an upper electrode of the ferroelectric capacitor. The contact plug is formed by partially removing the hydrogen barrier film. The contact plug itself is not a hydrogen barrier material, a part of its surrounding area is contacted to the interlevel insulator, and thus the contact plug is not entirely covered with the hydrogen barrier film. Accordingly, there is a possibility that hydrogen will penetrate the ferroelectric capacitor through the contact plug portion to degrade the characteristics of the ferroelectric capacitor.
Therefore, there is need for a semiconductor memory device which prevents the penetration of hydrogen or moisture to a ferroelectric capacitor from its surrounding area including a contact plug portion, and its manufacturing method.